GPS receiver having an initial adjustment for correcting for drift in reference frequency

ABSTRACT

A GPS receiver having a rapid acquisition of a GPS satellite signal when a normal operational mode is entered after a low power standby mode. The GPS receiver includes an RF section for receiving the GPS satellite signal and providing an GPS IF signal, a correlator section for providing a correlation signal for the correlation between the GPS IF signal and an internally generated replica signal, and a microprocessor section for receiving the correlation signal and calculating a geographical location of the GPS receiver. The replica signal is based upon a reference frequency from a reference oscillator and a reference time of arrival (TOA) from a timer. In order to increase acquisition speed, the microprocessor section provides the correlator section with an initial frequency adjustment and an initial TOA adjustment to correct for drift in the reference frequency during the standby mode. The initial adjustments are based upon a learned corrections to the initial adjustments that result in acquisition of the GPS satellite signal after alternating one or more times between the standby mode and the normal mode or from stored temperature relationships and measured temperatures of the reference oscillator.

DIVISIONAL APPLICATION

This application is a division of an application Ser. No. 08/332,958,U.S. Pat. No. 5,594,453 filed Nov. 1, 1994 by the same inventors andassigned to the same assignee.

BACKGROUND OF THE INVENTION Cross Reference to Related Application

This application is related to an application of Chung Y. Lau et al.Ser. No. 8-276886, filed Jul. 18, 1994 assigned to the same assignee asthe present application.

1. Field of the Invention

The invention relates generally to Global Positioning System receiversand more particularly to a Global Positioning System receiver having arapid acquisition of GPS satellite signals.

2. Background of the Invention

A Global Positioning System (GPS) now provides a worldwide, 24 hour,location service. The system includes multiple GPS satellites tobroadcast location signals, control stations to monitor and control thesatellites, and GPS receivers to receive the signals. Commercial GlobalPositioning System (GPS) receivers now are used to provide accuratelocation information in many navigation, tracking, and timingapplications. A GPS antenna that is a part of a GPS receiver must have aline of sight to a GPS satellite to receive the GPS signal from thatsatellite.

GPS location is based on one-way ranging from the GPS satellites to theGPS antenna. Ranges are measured to four satellites simultaneously inview by matching (correlating) the frequency and the time of arrival(TOA) of the incoming GPS signal to a receiver-generated replica signal.With four ranges, the receiver can determine four unknowns, typicallylatitude, longitude, altitude, and an adjustment to the replica. Theranges are called "pseudoranges" because the actual distances to the GPSsatellite are not known until the internal replica has been adjusted.Time of day is computed from the adjustment to the TOA of the replica.

Each GPS satellite broadcasts its position in a signal having a carrierfrequency at approximately 1.575 GHz. The signal is modulated by a PRNcode sequence of 1023 chips, repeating (arriving) at a 1 millisecondtime interval, where each satellite uses a different PRN code sequence.The use of the different PRN sequences enables a GPS receiver todistinguish the GPS signals from different GPS satellites. The frequencyof the signal received from each GPS satellite will have a Doppler shiftdue to the relative velocity between the GPS satellite and a GPSantenna. A velocity for the GPS antenna may be determined from the rateof change of the location or from the rate of change of the pseudorangesafter accounting for the Doppler shift due to the motion of thesatellite.

Power consumption is an important figure of merit for a GPS receiver. Alow power consumption is good for a GPS receiver that depends upon abattery for a power source. To achieve low power, some GPS receivershave a low power standby mode, where power consumption is reduced butthe GPS signals are not tracked. Some GPS receivers have a system toalternate between a normal mode to obtain a location fix and the lowpower standby mode.

Another important figure of merit for a GPS receiver is the timerequired to obtain a location fix or the "acquisition time." A fastacquisition is good because a user does not need to wait as long for anew location fix. In GPS receivers that automatically cycle to power offor to the standby mode between location fixes, a fast acquisition timeis good because less time is used in the normal mode to acquire the GPSsignal, resulting in a lower average power consumption.

Three acquisition times are often quoted. A "time to first fix,"sometimes called a "cold start" acquisition, refers to the time toacquire the GPS satellite signal and obtain a location fix when the GPSreceiver has not had a location fix within the previous few hours. A"signal interruption" acquisition time refers to the time to reacquirethe GPS satellite signal and to obtain a location fix after the line ofsight to the GPS satellite signal has been blocked and then unblocked. A"time to subsequent acquisition," sometimes called a "warm start"acquisition, refers to the time to reacquire the GPS satellite signaland obtain a location fix, when the GPS receiver has had a location fixwithin the previous few hours.

The subsequent acquisition is fast when the initial replica frequency iswithin 500 Hertz (0.3 pans per million) of the GPS frequency and theinitial replica TOA is within 500μs of the GPS TOA. Typically, thegreater the differences between the initial replica frequency and theGPS frequency and between the initial replica TOA and the GPS TOA, thegreater the time required to acquire the GPS signal.

Some GPS receivers reduce the difference between the initial replicafrequency and the GPS frequency by synchronizing the replica frequencyto a reference oscillator that continues to operate during a standbytime duration. However, a change in temperature inside the receiver,because less heat is being generated during the standby mode or/andbecause the outside temperature changes, will probably cause thereference oscillator frequency to drift, thereby causing the replicafrequency to drift. An inexpensive XO reference oscillator, using acrystal as a resonator, provides a frequency stability of approximately5 to 50 parts per million (ppm) within a temperature range of -40° C. to+85° C. A problem with an XO is that a change in temperature of 1° C. orless typically causes a change in frequency of more than 0.3 ppm. Atemperature compensated crystal oscillator (TCXO) may be used to improvethe stability to approximately 0.5 to 5 ppm within -40° C. to +85° C.but still may not be stable enough to provide 0.3 ppm during a typicalstandby time duration. A further problem with a TCXO is that theincremental cost of a TCXO over an XO is a significant pan of the costof an entire GPS receiver. A temperature stabilized oven, enclosing thereference oscillator, further improves the frequency stability but theoven would have to remain on, requiring a large power consumption duringthe standby mode in order to provide the improvement. A further problemis that an oven stabilized oscillator may cost as much or more than anentire GPS receiver.

What is needed is a GPS receiver having a rapid acquisition of GPSsatellite signals, after a time duration in a low power standby mode,using a reference oscillator having a frequency variation of up to 50ppm over the range of -40° C. to +85° C.

SUMMARY OF THE PRESENT INVENTION

It is therefore an object of the present invention to provide a GPSreceiver having a rapid acquisition of GPS satellite signals following atime duration in a low power standby mode, using an internal GPS replicasignal that is derived from a reference oscillator having a frequencyvariation of up to 50 ppm.

Another object is to provide a GPS receiver where a time of arrival(TOA) of a PRN code of the GPS replica signal is derived from areference timer that retains synchronization to the reference oscillatorduring the standby mode.

Another object is to provide a GPS receiver using a learned frequencycorrection to compensate for drift in the reference frequency thatoccurs during the standby time duration by correcting an initialfrequency adjustment of the replica signal used to initiate theacquisition of the GPS signal following the standby time duration.

Another object is to provide a GPS receiver using a learned TOAcorrection to compensate for drift in the reference TOA that occursduring the standby time duration by correcting an initial reference TOAadjustment used to initiate the acquisition of the GPS signal followingthe standby time duration.

Another object is to provide a GPS receiver using a frequencycorrection, based upon a temperature of the reference oscillator and afrequency/temperature relationship for the reference oscillator, tocompensate for drift in the reference frequency that occurs during thestandby time duration by correcting the initial frequency adjustment ofthe replica signal used to initiate the acquisition of the GPS signalfollowing the standby time duration.

Another object is to provide a GPS receiver using a TOA correction,based upon an elapsed time in the standby time duration and aTOA/temperature relationship, to compensate for drift in the referenceTOA that occurs during the standby time duration by correcting theinitial TOA adjustment used to initiate the acquisition of the GPSsignal following the standby time duration.

Briefly, the preferred embodiment of the present invention is a GPSreceiver, having normal mode where GPS satellite signals are acquiredand tracked and GPS location information is provided, having a low powerstandby mode where GPS satellite signals are not tracked, and having arapid subsequent acquisition of GPS satellite signals when the GPSreceiver enters the normal mode after a time duration in the standbymode. In the normal mode, an RF section receives a GPS satellite signaland provides an IF signal to a correlation section to provide acorrelation signal to a microprocessor section for calculating a GPSlocation from information included in the GPS satellite signal. In thestandby mode, the operation is inhibited and the power consumption issubstantially eliminated in the RF section, the correlator section, andthe microprocessor section. A reference oscillator and a timer continueto provide a reference frequency and a reference time of arrival (TOA)during the low power standby mode. The GPS receiver acquires the GPSsignal by adjusting a replica signal, based upon the reference frequencyand the reference TOA, to correlate to the GPS signal. The acquisitionis fast when the initial adjustment to the replica provides correlationwith no or few additional adjustment cycles. Two preferred embodimentsare disclosed to correct the replica signal for drift in the referencefrequency and in the reference TOA during the standby mode. In a firstembodiment, a selected time duration in the normal mode alternates witha selected time duration in the standby mode. The reference frequencydrift and the reference TOA drift are learned and used to correct theinitial adjustment following the next standby mode. In a secondembodiment, a temperature sensor measures a temperature during a lastlocation fix before entering the standby mode and measures a temperaturewhen the GPS receiver re-enters the normal mode. A frequency/temperaturerelationship and a TOA temperature relationship are pre-determined andstored in memory. A reference frequency drift correction and a referenceTOA drift correction are calculated from the relationships and used tocorrect the initial adjustment following a standby mode.

An advantage of the present invention is that the GPS receiver uses areference oscillator having a frequency variation of up to 50 ppm tohave a rapid subsequent acquisition of GPS satellite when the GPSreceiver turns on following a time duration in the standby mode.

Another advantage is that the GPS receiver uses a timer that retainssynchronization to the reference oscillator during the standby timeduration.

Another advantage is that the GPS receiver learns a frequency correctionto correct for drift that occurs in the reference frequency during thestandby time duration.

Another advantage is that the GPS receiver learns a TOA correction tocorrect for drift that occurs in the reference TOA during the standbytime duration.

Another advantage is that the GPS receiver uses the temperature of thereference oscillator and a stored frequency/temperature relationship tocorrect for drift that occurs in the reference frequency during thestandby time duration.

Another advantage is that the GPS receiver uses an elapsed time in thestandby mode and a stored TOA/temperature relationship to correct fordrift that occurs in the reference TOA during the standby time duration.

These and other objects and advantages of the present invention will nodoubt become obvious to those skilled in the art after having read thefollowing detailed description of the preferred embodiment.

IN THE DRAWINGS

FIGS. 1A and 1B are a block diagram of a GPS receiver having a rapidsubsequent acquisition of GPS satellite signals according to the presentinvention;

FIG. 2 is a curve of the frequency/temperature relationship of thereference frequency in the GPS receiver in FIG. 1;

FIG. 3 is a flow chart diagram of a method using learned corrections forthe subsequent acquisition of the GPS satellite signals by the GPSreceiver of FIG. 1,

FIG. 4 is a flow chart diagram of a method using thefrequency/temperature relationship of FIG. 2 for the subsequentacquisition of the GPS satellite signals by the GPS receiver of FIG. 1;and

FIG. 5 is a table for the frequency/temperature relationship of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a GPS receiver 10 to acquire a GPS satellite signalfrom one or more GPS satellites and to use information in the GPS signalto provide a location, a time of day, and a directional velocity for theGPS receiver 10. The GPS signal has a carrier frequency of approximately1.575 GHz modulated by a PRN C/A code sequence repeating at a 1millisecond time interval. The GPS receiver 10 computes the location andthe time of day from the time of arrival (TOA) of the PRN C/A codesequences in the GPS signal as compared to the time of arrival (TOA) ofan internally generated PRN C/A code sequence. The PRN C/A code sequencein the GPS signal includes ephemeris information for the location inspace of the GPS satellite and almanac information for the approximatelocations in space of all the other GPS satellites. Each GPS signal usesa distinct PRN code sequence to enable the GPS receiver 10 todistinguish the GPS signals from each of the GPS satellites. The PRNcodes and the format of the C/A information are set forth in GPSInterface Control Document ICD-GPS-200, published by RockwellInternational Division, Revision A, 26 September 1984, which isincorporated by reference herein.

The GPS receiver 10 includes an RF section 13 including a GPS antenna 12to receive the GPS satellite signal and to issue an antenna outputsignal, a local oscillator system 15 to provide one or more localoscillator signals, and a frequency downconverter 14 to receive theantenna output signal and to use the local oscillator signal to shiftthe frequency of the antenna output signal to an intermediate frequency(IF), signal. The IF signal has a frequency that represents thefrequency of the GPS satellite signal and a TOA that represents the TOAof the PRN C/A code sequence of the GPS satellite signal. A correlatorcircuit 16, including a carrier correlator 18 and a code correlator 19,receives the IF signal from the frequency downconverter 14. A referenceoscillator 21 provides a reference frequency in a signal. The carriercorrelator 18 receives the reference frequency signal, adjusts thereference frequency to generate a replica signal frequency, and providesa correlation signal for the correlation between the IF signal frequencyand the replica signal frequency. A timer 22 receives the referencefrequency signal and provides a reference TOA in a signal. The codecorrelator 19 receives the reference TOA signal, adjusts the referenceTOA to generate a replica signal PRN code TOA, and provides acorrelation signal for the correlation between the IF signal PRN codeTOA and the replica signal PRN code TOA. The PRN code sequence in thereplica signal code is either stored or generated to match the PRNsequence of the GPS satellite to be acquired or tracked. The localoscillator system 15 receives the reference frequency signal in order toprovide the local oscillator signal to the frequency downconverter 14.

A microprocessor section 24 receives the carrier correlation signal,calculates the frequency adjustment to obtain the correlation betweenthe replica signal frequency and the IF signal frequency, and providesthe adjustment to the carrier correlator 18 in a feedback adjustmentsignal. The microprocessor section 24 receives the code correlationsignal, calculates the TOA adjustment to obtain the correlation betweenthe replica signal code TOA and the IF signal code TOA, and provides theadjustment to the code correlator 19 in a feedback adjustment signal.One or more frequency adjustments are provided in the feedbackadjustment signal, where each frequency adjustment corresponds to one ofthe one or more GPS satellite signals that are to be acquired or arebeing tracked. One or more TOA adjustments are provided in the feedbackadjustment signal, where each TOA adjustment corresponds to one of theone or more GPS satellite signals that are to be acquired or are beingtracked. The GPS satellite signal is acquired by adjusting the replicafrequency until correlation to the IF signal frequency is obtained andadjusting the replica signal TOA until correlation to the IF signal TOAis obtained. When the GPS satellite signals are being tracked, thereplica signal frequency and the replica signal TOA are beingcontinuously adjusted to maintain correlation. The carrier correlationand the code correlation are performed approximately simultaneously.

A psuedorange for the GPS satellite signal is measured from the replicasignal code TOA that correlates to the IF signal code TOA. With fourpsuedoranges and the locations in space of the associated GPSsatellites, the GPS microprocessor section 24 computes time of day and 3spatial dimensions of a location, such as: x distance, y distance, and zdistance from a reference location point, or a latitude, a longitude, analtitude. Location may be computed with fewer than four psuedorange ifthe microprocessor section 24 is given information for an altitude or aprecise time of day from an external source. A psuedorange rate ismeasured from the replica signal frequency that correlates to the IFsignal frequency. With four psuedorange rates, the microprocessorsection 24 computes a directional velocity. Fewer than four psuedorangerates may be needed if a speed, a direction, or a precise frequency isavailable from another source.

The microprocessor section 24 includes a microprocessor 25 capable ofexecuting program instructions in an executable program 26. Theexecutable program 26 and pre-determined information are stored in aprogram memory 27. A variable memory 28 stores variable information. Theprogram memory 27 and the variable memory 28 have the capability ofretaining stored instructions and information when the GPS receiver 10is powered off or in a standby mode. The program memory 27 is anerasable, programmable read only memory (EPROM). Other memory types,including masked ROM, one time programmable (OTP) ROM, flash memory, orbattery backed static random access (SRAM), could be used. The variablememory 28 is battery backed up SRAM. Other memory types, including flashmemory, could be used. The microprocessor 25 operates in a conventionalmanner to receive digital signals representing information, to processthe information by executing instructions in the executable program 26,and to issue digital signals representing information to control theelements in the microprocessor section 24 and in the GPS receiver 10.

The reference oscillator 21 uses an AT cut crystal resonator to providethe reference frequency such as approximately 16.368 MHz, approximately10 MHz, or approximately 12.5 MHz. The choice of reference frequency isnot critical to the present invention. The timer 22 generates thereference TOA signal by counting a pre-determined number of cycles ofthe reference frequency signal received from the reference oscillator 21to cause the time interval of the reference TOA signal to approximatelymatch the time interval of the GPS signal code of 1 millisecond.

The GPS receiver 10 has a fully powered, normal mode where the GPSsatellite signals are acquired and tracked and GPS location informationis provided, and a low power standby mode, where the GPS satellitesignals are not tracked. In the standby mode, the operation is inhibitedand the power consumption is reduced in the RF section 13, thecorrelator section 16, and the microprocessor section 24. The modes ofeach of the RF section 13, the correlator section 16, and themicroprocessor section 24 are separately controllable by themicroprocessor section 24.

An internal or an external power supply provides power for the GPSreceiver 10. The mode of the RF section 13 is controlled by a powercontroller 31 to control the flow of power to the RF section 13. In thestandby mode, the microprocessor section 24 executes programinstructions in the executable program 26 to control the powercontroller 31 to inhibit the flow of power to the RF section 13 in thelow power standby mode and to pass power to the RF section 13 in thenormal mode.

The mode of the correlator section 16 and the mode of the microprocessorsection 24 are controlled by inhibiting the respective clock signalsdriving those sections. The circuits in the correlator section 16 andthe microprocessor section 24 are designed with Complementary MetalOxide Silicon (CMOS) that operate synchronously with a clock signal.These circuits consume power primarily when changing digital states.When the CMOS circuits do not receive a clock signal, the digital statesof the circuits are retained and the power consumed in the circuits isreduced by at least an order of magnitude. A correlator clock enableregister 33 controls the flow of the clock signal to the correlatorsection 16. The microprocessor section 24 executes program instructionsin the executable program 26 to control the correlator clock enableregister 33 to inhibit the flow of the clock signal in the correlatorsection 16 in the low power standby mode and to pass the clock signal tothe correlator section 16 in the normal mode.

The microprocessor section 24 includes a microprocessor clock 37 toprovide a microprocessor clock signal. Alternatively, the microprocessorclock signal is provided by another source. The microprocessor 25 in thepreferred embodiment is a static microprocessor of the CPU 32 familymanufactured by Motorola having a capability of executing an "LPstop" toinhibit the microprocessor clock 37 or/and a clock signal received fromanother source and including an interrupt capability to cause it to passthe clock signal upon the receipt of a wakeup interrupt signal. Themicroprocessor section 24 executes LPstop in the executable program 26to inhibit the clock signal to enter the low power standby mode andreceives the wakeup interrupt signal to re-enter the normal mode.

A real time clock 38, including an RTC clock source, provides the wakeupinterrupt to the microprocessor section 24 at the completion of aselected standby time duration. The microprocessor section 24 selectsthe time duration and initializes the real time clock 38 with a digitalcontrol signal. The real time clock 38 also provides time of dayinformation to the microprocessor section 24 in a time signal. Themicroprocessor section 24 uses the time of day information in theestimation of the locations in space of the GPS satellites in order toestimate the initial frequency adjustment and the initial TOA adjustmentused for the subsequent acquisition.

When the GPS receiver 10 enters the normal mode after a time duration inthe standby mode, the microprocessor section 24 executes instructions inthe executable program 26 to estimate new adjustments to the replicafrequency and to the replica TOA, based upon the new time and upon thenew locations of GPS satellites and of the GPS receiver 10, for thesubsequent acquisition of the GPS satellite signal. The adjustments areprovided to the carrier correlator 18 and to the code correlator 19 forthe initial frequency and initial TOA of the replica for the searchesfor the GPS IF frequency and for the GPS IF TOA, respectively.Subsequent acquisition occurs most rapidly where the frequency and theTOA of the initial replica most closely approximate the frequency andthe TOA of the GPS IF signal, respectively. When the frequency of thereplica matches the frequency of the GPS signal to within approximately500 Hertz referred to the GPS carrier frequency (0.3 ppm), and the TOAof the replica matches the TOA of the GPS signal to within approximately500 us, the acquisition time is fast because no or few additionaladjustments are required to obtain correlation. The reference oscillator21, the timer 22, and the real time clock 38 receive power and continueto operate during the low power standby mode in order to provide thereference frequency, the reference TOA, and the time of day for use insubsequent acquisition.

A means for controlling the mode of the GPS receiver 10 includes thepower controller 31 to control the flow of power to the RF section 13,the correlator clock enable register 33 to control the flow of clock tothe correlator circuit 16, the real time clock 38 to provide elapsedtime information and to provide a wakeup interrupt, the executableprogram 26 to retain program instructions for the LPstop and theselection of the mode, and the microprocessor 25 to receive the wakeupinterrupt, and to execute the program instructions to control the powercontroller 31, the correlator clock enable register 33, themicroprocessor clock 37, and the real time clock 38.

The circuits in the GPS receiver 10 are housed in a package. Thetemperature inside the package can change due to a change in thetemperature outside of the package or due to a change in a temperaturedifference between the inside and the outside of the housing. Typically,the temperature inside the GPS receiver 10 decreases during the timeduration in the low power standby mode because less heat is generated bythe few circuits operating during the standby mode than by all thecircuits operating during the normal mode. When the GPS receiver 10 istracking the GPS signal, the replica frequency and the replica TOA arecontinuously adjusted by the microprocessor section 24 to correlate tothe IF signal, thereby correcting for a reference frequency drift due toa change in temperature. During the time duration when the GPS receiver10 is in the standby mode the reference frequency probably driftscausing the initial replica frequency and causing the initial replicaTOA used in the subsequent acquisition to have an error relative to theIF frequency and TOA, respectively.

In a first preferred embodiment, the microprocessor 25 executes theexecutable program 26 to cause the GPS receiver 10 to alternate betweena selected time duration in the normal mode and a selected time durationin the standby mode. The selected time durations are pre-determined,based upon the intended application, or determined by a user of the GPSreceiver 10 and passed to the microprocessor section 24 in a user inputsignal. Because the time durations are repeated, a frequency correctionthat compensates the drift is approximately the same for each subsequentacquisition. The frequency correction is stored in the variable memory28 as a learned frequency correction 40. Similarly, a TOA correctionthat compensates for the drift is approximately the same for eachsubsequent acquisition. The TOA correction is stored in the variablememory 28 as a learned TOA correction 41. When the GPS receiver 10re-enters the normal mode after a time duration in the standby mode, theexecutable program 26 uses the learned frequency correction 40 and thelearned TOA correction 41 to correct the initial frequency adjustmentand the initial TOA adjustment, respectively, provided to the correlatorsection 16. FIG. 3 describes the steps in a method, whereby the GPSreceiver 10 has a rapid acquisition of the GPS signal according to thefirst embodiment.

In a second preferred embodiment, the GPS receiver 10 includes atemperature sensor 39 to provide a temperature signal having informationfor the temperature of the reference oscillator 21 inside the package ofthe GPS receiver 10. A frequency/temperature relationship 42 ispre-determined in design and included in the program memory 27.Optionally, the frequency/temperature relationship 42 may bepre-determined for each unit by testing. Testing each unit gives a moreaccurate relationship but is more expensive in manufacturing. Themicroprocessor 25 executes the executable program 26 to apply thetemperature information to the frequency/temperature relationship 42 toestimate the frequency correction required to compensate for the driftcaused by a change in temperature that occurred during the standby timeduration. Similarly, a TOA/temperature relationship 43 is pre-determinedin design and included in the program memory 27. Optionally, theTOA/temperature relationship 43 may be pre-determined for each unit bytesting. Testing each unit gives a more accurate relationship but ismore expensive in manufacturing. The microprocessor 25 executes theexecutable program 26 to apply the frequency correction, the elapsedtime between a time of day of a last location fix and a time of day of asubsequent acquisition to the TOA/temperature relationship 43 toestimate the TOA correction required to compensate for the drift causedby a change in temperature that occurred during the standby timeduration. FIG. 4 describes the steps in a method, whereby the GPSreceiver 10 has a rapid acquisition of the GPS receiver 10 according tothe second embodiment.

FIG. 2 illustrates a curve 50 for the frequency/temperature relationship42 between the reference frequency and the temperature of the referenceoscillator 21. The vertical axis shows variation in the referencefrequency divided by the nominal frequency, Δf/f, in units of parts permillion (ppm). The horizontal axis shows temperature in degreesCentigrade (° C.). The curve 50 shows a frequency/temperaturerelationship 42 having a lower minimum point 51 at approximately -40°C., -7.5 ppm, a lower maximum point 52 at approximately -8° C., +6.1ppm, a mid point 53 at approximately+24° C., 0 ppm, an upper minimumpoint 54 at approximately+61° C., -7.9 ppm, and an upper maximum point55 at approximately +85° C., -1.6 ppm. A reference oscillator 21 mayhave a minimum and a maximum slightly smaller than shown in curve 50 or,using a less expensive crystal resonator, may have a minimum or/and amaximum up to plus or minus 50 ppm.

FIG. 3 illustrates a flow chart of the steps in a method for a rapidtime to subsequent acquisition according to the first preferredembodiment of the invention. The steps in the method are implemented bythe microprocessor section 24 by executing program instructions in theexecutable program 26 stored in the program memory 27. The GPS receiver10 alternates between a selected time duration in the normal mode and aselected time duration in the low power standby mode. Defaults for theselected time durations are pre-determined and stored in the programmemory 27 based upon the intended application of the GPS receiver 10. Auser can modify the selected time durations and cause the modified timesdurations to be stored in the variable memory 28. Typically, a locationfix is obtained during the normal mode. The GPS receiver 10 storesstatus information associated with a last location fix in the variablememory 28 including: (i) the spatial location coordinates of the GPSantenna 12, (ii) the directional velocity of the GPS antenna 12, (iii)the time of day, (iv) the ephemeris or/and almanac location informationfor the GPS satellite being tracked, (v) the learned frequencycorrection 40, and (vi) the learned TOA correction 41. The calculationsof the spatial location and the directional velocity use a coordinatesystem such as latitude, longitude, and altitude, or x, y, and zdistances from a reference location point, to describe geographicallocations in the vicinity of the surface of the Earth. When the GPSreceiver 10 calculates location and velocity using x, y, and zcoordinates, the coordinates are typically convened to another system,such as latitude, longitude, and altitude for display to a human user.FIG. 3 starts when the GPS receiver 10 re-enters the normal mode andbegins the subsequent acquisition of the GPS satellite signals.

Step 60 retrieves the status information associated with the lastlocation fix. Step 61 retrieves the learned frequency correction 40 andthe learned TOA correction 41. Step 62 receives a new time of dayprovided from the real time clock 38. Step 64 uses equations 1, 2, and 3to estimate a new location for the GPS antenna 12 based upon the lastlocation, upon the last directional velocity, and upon the elapsed timebetween the new time and the last time. Equation 1 solves for a newestimated location in the x dimension, x_(e), from the last location inthe x dimension, x_(I), and from the last velocity in the x direction,(dx/dt)_(l), multiplied by the elapsed time, Δt, from the last time tothe next time. Similarly, equations 2 and 3 solve for new estimatedlocations in the y dimension and the z dimension.

    x.sub.e =x.sub.l +(dx/dt).sub.l Δt                   (1)

    y.sub.e =y.sub.l +(dy/dt).sub.l Δt                   (2)

    z.sub.e =z.sub.l +(dz/dt).sub.l Δt                   (3)

where

x_(e) is the estimate of the new location in the x dimension

y_(e) is the estimate of the new location in the y dimension

z_(e) is the estimate of the new location in the z dimension

x_(I) is the last location in the x dimension

y_(l) is the last location in the y dimension

z_(l) is the last location in the z dimension

(dx/dt)l is the last velocity in the x direction

(dy/dt)l is the last velocity in the y direction

(dz/dt)l is the last velocity in the z direction Δt At is the elapsedtime

Optionally, step 64 may include only a part or none of the last velocityin the estimation of the new location. Step 65 estimates the newvelocity to be the last velocity. Optionally, step 65 may include only apart or none of the last velocity in the estimate of the new velocity.Step 66 estimates a new location in space for the GPS satellite basedupon the ephemeris or/and the almanac information and the new time ofday. Step 67 estimates the replica frequency adjustment, accounting forthe relative Doppler shift between the GPS satellite and the GPS antenna12, based upon the new time, the estimated new location, the estimatednew directional velocity, the estimated location in space of the GPSsatellite, and the equations for the GPS satellite orbital parameters.Step 68 estimates the initial TOA adjustment, based upon the estimatednew location, the estimated location in space of the GPS satellite, andthe equations for the GPS adding the learned frequency correction 40 tothe adjustment and provides the corrected initial frequency adjustmentto the carrier correlator 18. Step 72 corrects the initial replica TOAadjustment by adding the learned TOA correction 41 to the initialadjustment and provides the corrected initial TOA adjustment to the codecorrelator 19. A user is given the capability of interrupting thestandby mode before the standby time duration is completed. When thestandby mode is interrupted, the steps 70 and 72 correct the initialreplica frequency adjustment and correct the initial replica TOAadjustment, respectively, by the portion of the learned frequencycorrection 40 equivalent to the portion of the originally selectedstandby time duration that was completed before the interruption. Instep 74 the initial replica frequency and the replica TOA are adjustedto acquire the GPS signal by obtaining correlation. In step 76 thedifference between the corrected initial frequency adjustment providedin step 70 and the frequency adjustment that resulted in correlation isused to calculate a new learned frequency correction 40 according toequation 4. Equation 4 uses a weighting factor, k_(f), to merge theactual frequency correction C_(fa) and the existing learned frequencycorrection, LFC_(e) to obtain the new learned frequency correctionLFC_(n). The new learned frequency correction, LFC_(n) is stored in thevariable memory. 28 as the learned frequency correction 40. In step 78,the difference between the corrected initial TOA adjustment provided instep 72 and the TOA adjustment that resulted in correlation is used tocalculate a new learned TOA correction LFC_(n) according to equation 5.Equation 5 uses a weighting factor, k_(t), to merge the actual TOAcorrection C_(ta) and into the existing learned TOA correction LTC_(e)to obtain the new learned TOA correction LTC_(n). The new learned TOAcorrection LTC_(n) is stored in the variable memory 28 as the learnedTOA correction 41.

    LFC.sub.n =k.sub.f *C.sub.fa +(1-k.sub.f)*LFC.sub.e        (4)

    LTC.sub.n =k.sub.t *C.sub.ta +(1-k.sub.t)*LTC.sub.e        (5)

where

LFC_(n) is the new learned frequency correction

C_(fa) is the actual frequency correction to obtain correlation

LFC_(e) is the existing learned frequency correction

LTC_(n) is the new learned TOA correction

C_(ta) is the actual TOA correction to obtain correlation

LTC_(e) is the existing learned TOA correction

k_(f) is a selected frequency weighting factor, 0<k_(f) ≦1

k_(t) is a selected TOA weighting factor, 0<k_(t) ≦1

The weighting factors, k_(f) and k_(t) are selected at 0.1. Anyweighting factor greater than 0 and less than or equal to 1 will improvethe speed of the acquisition. A weighting factor of 0 has no learning. Aweighting factor of 1 has no memory. Step 80 stores the statusinformation associated with the location fix. If no location is obtainedafter the selected time duration for the normal mode, the existinglearned correction information and the existing status information isretained and the GPS receiver 10 goes into the standby mode.

FIG. 4 illustrates a flow chart of the steps in a method for a rapidtime to subsequent acquisition according to the second preferredembodiment of the invention. The GPS receiver 10 stores statusinformation associated with a location fix in the variable memory 28including: (i) the spatial location coordinates of the GPS antenna 12,(ii) the directional velocity of the GPS antenna 12, (iii) the time ofday, (iv) the ephemeris or/and almanac location information for the GPSsatellite being tracked, (v) the reference frequency adjustment, (vi)the reference TOA adjustment, and (vii) the temperature of the referenceoscillator 21. The calculations of the spatial location and of thedirectional velocity use a coordinate system such as latitude,longitude, and altitude, or x, y, and z distances from a referencelocation point, to describe a geographical location in the vicinity ofthe surface of the Earth. When the GPS receiver 10 calculates thelocation and the velocity using x, y, and z coordinates, the coordinatesare typically converted to another system, such as latitude, longitude,and altitude for display to a human user. The steps in the method areimplemented by the microprocessor section 24 by executing programinstructions in the executable program 26 stored in the program memory27. FIG. 4 starts when the GPS receiver 10 re-enters the normal mode andbegins the subsequent acquisition of the GPS satellite signals

Step 100 retrieves the status information associated with the lastlocation fix. Step 101 receives a new temperature from the temperaturesensor 39. Step 102 receives a new time of day from the real time clock38. Step 104 estimates a new location for the GPS antenna 12 based uponthe last location, the last directional velocity, and upon the elapsedtime between the new time and the last time according the equations in xdistance, y distance, and z distance from a reference location pointaccording to equations 1, 2, and 3. Optionally, step 104 may includeonly a part or none of the last velocity in the estimation of the newlocation. Step 105 estimates the new velocity to be the last velocity.Optionally, step 105 may include only a part or none of the lastvelocity in the estimate of the new velocity. Step 106 estimates a newlocation in space for the GPS satellite based upon the ephemeris or/andalmanac information and upon the new time. Step 107 estimates theinitial replica frequency adjustment, based upon the last frequencyadjustment and upon the relative Doppler shift between the GPS satelliteand the GPS antenna. The relative Doppler shift is calculated from theestimated new location of the GPS antenna 12, the estimated newdirectional velocity of the GPS antenna 12, the estimated location inspace of the GPS satellite, and the equations for the GPS satelliteorbital parameters. Step 108 estimates the replica TOA adjustment, basedupon the last TOA adjustment and upon the range to the GPS satellite.The relative range is calculated from the estimated new location of theGPS antenna 12, the estimated location in space of the GPS satellite,and the equations for the GPS satellite orbital parameters.

Step 110 estimates a frequency correction, based upon the lasttemperature retrieved in step 100, the new temperature received in step101, and the frequency/temperature relationship 42. The information inthe relationship 42 can be stored in a table, in coefficients of anequation, or both in a table and in coefficients. The coefficients maybe calculated from the table using linear algebra or the table may becalculated from the coefficients by solving algebraic equations. Eitherthe table or the coefficients can be used to calculate the frequencyvariation. FIG. 5 illustrates the table for the pre-determinedfrequency/temperature relationship 42. The table in FIG. 5 shows atemperature in degrees Centigrade and the variation in the referencefrequency in parts per million (ppm) associated with the temperature. Tocalculate a frequency variation using the table, a table lookupprocedure finds the last frequency variation associated with the lasttemperature, finds the new frequency variation associated with the newtemperature, and subtracts the last frequency variation from the newfrequency variation. The resulting frequency variation in ppm ismultiplied by the nominal frequency of the GPS signal. The frequencycorrection to the frequency adjustment to the replica signal frequencyis the negative of the frequency variation. As an example using thetable 1, assume the last temperature was 30° C. and the new temperatureis 25° C. The last frequency variation is -1.8 ppm, the new frequencyvariation is -0.3 ppm, so the difference is 1.5 ppm. Linearinterpolation is used when a measured temperature is not identical to atemperature included in the table. Multiplying 1.5 ppm by the GPSfrequency of 1.575 GHz gives a frequency variation of approximately 2400Hertz. The frequency correction is the negative of the variation or -1.5ppm and -2400 Hertz referred to the GPS frequency.

Equation 6 illustrates a fifth order equation to approximate thefrequency/ temperature relationship 42.

    Δf/f.sub.e =a.sub.0 +a.sub.1 T+a.sub.2 T.sup.2 +a.sub.3 T.sup.3 +a.sub.4 T.sup.4 +a.sub.5 T.sup.5                         (6)

where:

T is the temperature

Δf/f_(e) is the variation in the reference frequency in ppm

a_(O), a_(l), a₂, a₃, a₄, a₅ are the coefficients of thefrequency/temperature relationship

The coefficients a₀, a₁, a₂, a₃, a₄, and a₅ are determined in design ormanufacturing by measuring a frequency error at six temperatures andsolving the resulting matrix of six linear equations. The coefficientsa₀ =5.70 E0, a₁ =-1.02 E-1, a₂ =-7.81 E-3, a₃ =8.82 E-5, a₄ =2.49 E-7,and a₅ =-2.12 E-9 give a satisfactory agreement of 0.1 ppm over any 5degree Centigrade increment to the frequency/temperature relationship 42illustrated in the table in FIG. 5. Closer agreement could be obtainedby using more than six power terms for equation 6. To calculate afrequency variation using the coefficients, equation 6 is solved for alast frequency variation at the last temperature and a new frequencyvariation at the new temperature. The last frequency variation issubtracted from the new frequency variation and multiplied by thenominal frequency as in the example above. The frequency correction isthe negative of the variation.

Step 112 estimates the TOA correction, based upon the frequencycorrection from step 110, an elapsed time calculated from the last timeretrieved in step 100, the new time received in step 102, and theTOA/frequency relationship 43. Equation 7 illustrates a first orderequation for the TOA/temperature relationship 43.

    ΔTOA=k.sub.1 *Δf/f.sub.e *Δt             (7)

where

ΔTOA is the time of arrival (TOA) variation in microseconds

Δf/f_(e) is the frequency correction in the reference frequency in ppm

Δt is the time elapsed between the last time and the new time.

k₁ is the coefficient of the TOA/temperature relationship

The coefficient k_(l) has been pre-determined to be approximately 0.5.Step 112 calculates an estimation of the TOA variation by solving forΔTOA. As an example, assume a frequency correction of 5.0 ppm and anelapsed time of 300 seconds. Using the equation 7, the TOA correction is+750 us.

Step 114 corrects the initial replica frequency adjustment from step 107by adding the frequency correction estimated in step 110 and providesthe corrected adjustment in the initial frequency adjustment to thecarrier correlator 18. Step 116 corrects the initial replica TOAadjustment from step 108 by adding the TOA correction estimated in step112 and provides the corrected adjustment in the initial TOA adjustmentto the code correlator 19. In step 118 the frequency adjustment and theTOA adjustment are adjusted until correlation to the GPS signal isobtained. Step 120 stores the status information to be retrieved in step100.

Although the invention has been described in terms of the presentlypreferred embodiments, it is to be understood that such disclosure isnot to be interpreted as limiting. Various embodiments and modificationswill no doubt be apparent to those skilled in the art. Accordingly, itis intended that the appended claims be interpreted as coveringalterations and modifications as fall within the true spirit and scopeof the invention.

We claim:
 1. A global positioning system (GPS) receiver for receiving aGPS signal, including:a memory including an executable program forcalculating an initial time of arrival (TOA) adjustment to correct for aTOA drift caused by a drift in a reference frequency that has occurredduring a time duration in a standby mode; and a microprocessor coupledto the memory for using the executable program for providing saidinitial TOA adjustment.
 2. The GPS receiver of claim 1, wherein:thememory further includes a learned TOA correction; and said executableprogram is further for estimating a TOA adjustment for use in acquiringsaid GPS signal, correcting said TOA adjustment until said GPS signal isacquired, calculating said learned TOA correction based upon acomparison of said estimated and said corrected TOA adjustment, andcalculating said initial TOA adjustment based upon said learned TOAcorrection.
 3. The GPS receiver of claim 1, wherein:the memory furtherincludes a TOA/temperature relationship for relating said TOA drift to atemperature of a reference oscillator providing said referencefrequency; the microprocessor is further for receiving a signalindicative of said temperature; and said executable program is furtherfor calculating said initial TOA adjustment based upon saidTOA/temperature relationship and said temperature.
 4. A method in aglobal positioning system (GPS) receiver having a microprocessor coupledto a memory acquisition of a GPS signal after a time duration in astandby mode, including steps of:receiving said GPS signal with said GPSreceiver; providing a timer signal having a reference time of arrival(TOA) based upon a reference frequency for use in acquiring said GPSsignal; and calculating an initial TOA adjustment for correcting for aTOA drift in said reference TOA that has occurred during said timeduration in said standby mode.
 5. The method of claim 4, wherein:thestep of calculating an initial TOA adjustment includes steps ofestimating a TOA adjustment for use in acquiring said GPS signal;correcting said TOA adjustment until said GPS signal is acquired;calculating a learned TOA correction based upon a comparison of saidestimated and said corrected TOA adjustment; and calculating saidinitial TOA adjustment based upon said learned TOA correction.
 6. Themethod of claim 4, wherein:the step of calculating an initial frequencyadjustment includes steps of receiving data indicative of a temperatureof a reference oscillator providing said reference frequency; providinga temperature/frequency relationship in said memory relating saidreference TOA and said temperature; and calculating said initialfrequency adjustment based upon said temperature/frequency relationshipand said temperature.
 7. A global positioning system (GPS) receiver forreceiving a GPS satellite signal, including:a memory including anexecutable program for calculating an initial frequency adjustment tocorrect for a frequency drift that has occurred in a reference frequencyduring a time duration in a standby mode; a microprocessor coupled tothe memory for using the executable program for providing said initialfrequency adjustment; and a correlator for receiving a GPS intermediatesignal representative of said GPS satellite signal, receiving areference signal including said reference frequency and coupled to themicroprocessor for receiving said initial frequency adjustment, andproviding a correlation signal representative of a correlation betweensaid GPS intermediate signal and said reference signal as adjusted bysaid initial frequency adjustment when the GPS receiver enters anoperational mode after said time duration in said standby mode.
 8. TheGPS receiver of claim 7, wherein:the memory further includes a learnedfrequency correction; and said executable program is further forestimating a frequency adjustment for use in acquiring said GPS signal,correcting said frequency adjustment until said GPS signal is acquired,calculating said learned frequency correction based upon a comparison ofsaid estimated and said corrected frequency adjustment, and calculatingsaid initial frequency adjustment based upon said learned frequencycorrection.
 9. The GPS receiver of claim 7, wherein:the memory furtherincludes a frequency/temperature relationship for relating saidreference frequency to a temperature of said reference oscillator; themicroprocessor is further for receiving a signal indicative of saidtemperature; and said executable program is further for calculating saidinitial frequency adjustment based upon said temperature/frequencyrelationship and said temperature.
 10. A method in a global positioningsystem (GPS) receiver for improving the speed of acquisition of a GPSsatellite signal after a time duration in a standby mode, includingsteps of:receiving said GPS satellite signal with said GPS receiver;converting said GPS satellite signal to a GPS intermediate signal;receiving a reference signal having a reference frequency; calculatingan initial frequency adjustment for correcting for a drift in saidreference frequency that has occurred during said time duration in saidstandby mode with a microprocessor coupled to a memory having anexecutable code; and correlating said GPS intermediate signal with saidreference signal as adjusted by said initial frequency adjustment whensaid GPS receiver enters an operational mode after said time duration insaid standby mode.
 11. The method of claim 10, wherein:the step ofcalculating an initial frequency adjustment includes steps of providinga frequency adjustment that causes the GPS receiver to acquire said GPSsignal; calculating said learned frequency correction based upon saidfrequency adjustment; and calculating said initial frequency adjustmentfrom said learned frequency correction.
 12. The method of claim 10,wherein:the step of calculating an initial frequency adjustment includessteps of receiving a signal indicative of a temperature of a referenceoscillator providing said reference frequency; providing atemperature/frequency relationship in said memory relating saidreference frequency and said temperature; and calculating said initialfrequency adjustment based upon said temperature/frequency relationshipand said temperature.